netdev - Re: [PATCH v2] socket: Provide put_cmsg_whitelist() for constant size copies

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Date:   Wed, 7 Feb 2018 05:36:02 +1100
From:   Kees Cook <keescook@...omium.org>
To:     David Miller <davem@...emloft.net>
Cc:     syzbot <syzbot+e2d6cfb305e9f3911dea@...kaller.appspotmail.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Eric Biggers <ebiggers3@...il.com>,
        James Morse <james.morse@....com>, keun-o.park@...kmatter.ae,
        Laura Abbott <labbott@...hat.com>,
        Linux-MM <linux-mm@...ck.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v2] socket: Provide put_cmsg_whitelist() for constant size copies

On Wed, Feb 7, 2018 at 3:19 AM, David Miller <davem@...emloft.net> wrote:
> From: Kees Cook <keescook@...omium.org>
> Date: Tue, 6 Feb 2018 04:31:50 +1100
>
>> On Tue, Feb 6, 2018 at 2:03 AM, David Miller <davem@...emloft.net> wrote:
>>> From: Kees Cook <keescook@...omium.org>
>>> Date: Fri, 2 Feb 2018 02:27:49 -0800
>>>
>>>> @@ -343,6 +343,14 @@ struct ucred {
>>>>
>>>>  extern int move_addr_to_kernel(void __user *uaddr, int ulen, struct sockaddr_storage *kaddr);
>>>>  extern int put_cmsg(struct msghdr*, int level, int type, int len, void *data);
>>>> +/*
>>>> + * Provide a bounce buffer for copying cmsg data to userspace when the
>>>> + * target memory isn't already whitelisted for hardened usercopy.
>>>> + */
>>>> +#define put_cmsg_whitelist(_msg, _level, _type, _ptr) ({             \
>>>> +             typeof(*(_ptr)) _val = *(_ptr);                         \
>>>> +             put_cmsg(_msg, _level, _type, sizeof(_val), &_val);     \
>>>> +     })
>>>
>>> I understand what you are trying to achieve, but it's at a real cost
>>> here.  Some of these objects are structures, for example the struct
>>> sock_extended_err is 16 bytes.
>>
>> It didn't look like put_cmsg() was on a fast path, so it seemed like a
>> bounce buffer was the best solution here (and it's not without
>> precedent).
>
> For some things like timestamps it can be important.

Making put_cmsg() inline would help quite a bit with tracking the
builtin_const-ness, and that could speed things up a little bit too.
Would you be opposed to inlining?

-Kees

-- 
Kees Cook
Pixel Security